For the grain-finished leather-like sheets composed of a laminate of a fiber substrate and a resin surface layer thereon, the improvement of the surface strength is generally an important problem to be solved. The surface strength has been expressed by many terms according to the intended use of the products made from the grain-finished leather-like sheet. For example, the difficulty of wearing is expressed by a wear resistance, the difficulty of making recessed marks like embossed marks is expressed by a scratch resistance, and the difficult of making fine rubs by friction or irregularity due to degradation or discoloration is expressed by an abrasion resistance.
To improve the wear resistance, there have been used a method of forming the surface layer with a hard resin, a method of adding highly hard fine particles to the resin for the surface layer, a method of making the surface slippery by the modification of resin or the coating to change the surface to be hardly abraded, or a method of increasing the thickness of the surface layer to leave a large margin for wearing. The scratch resistance may be improved by forming the surface layer with a hard resin. The abrasion resistance has been improved by forming the surface layer with a hard resin or making the surface slippery. To make the surface slippery, a fine roughness has been formed on the surface to reduce the contact area with an object. Thus the methods of improving the abrasion resistance have been proposed also in a viewpoint different from the known method of improving the wear resistance by making the surface slippery. However, although the known methods improve the surface strength, the appearance, hand and touch of surface are likely impaired. Thus, the known methods are actually restricted largely with respect to the balance with the appearance and hand.
A leather-like sheet which has been improved in the wear resistance by making the surface resistant to wearing is proposed, for example, in Patent Document 1. In the proposed leather-like sheet, a natural leather-like substrate is laminated with a surface layer mainly made of a non-yellowing polycarbonate urethane resin containing spherical fine powders having a particle size of 10 μm or less which is extremely harder than the urethane resin. Examples of the spherical fine powder include inorganic spherical fine powders such as silica fine powder and glass fine powder, and crosslinked resin fine power such as crosslinked acrylic resin fine powder and crosslinked olefin resin fine powder. Another leather-like sheet which has been improved in the wear resistance by adding similar fine particles is disclosed, for example, in Patent Document 2. In the proposed leather-like sheet, a substrate is laminated with an outermost urethane coating layer containing hard fine particles having a Brinell hardness of 20 to 100. In the proposed techniques, the surface layer itself is made resistant to scaping off, thereby improving the wear resistance. However, since the compatibility is poor, the proposed urethane resin and the fine powder likely slip at their interface by external force. As a result, the plasticity of the urethane resin is increased apparently, thereby failing to improve the scratch resistance and rather reducing the scratch resistance in some cases. In addition, if the surface is slightly worn away to allow the hard fine particles to become exposed, the surface of an object which is in contact with such surface is likely to be scratched upon friction.
A leather-like sheet which is improved in the abrasion resistance by making its surface slippery has been proposed, for example, in Patent Document 3, in which a substrate is bonded to a crosslinked, silicone-modified polyurethane coating layer via a non-crosslinked polyurethane. In another proposed method, the abrasion resistance is improved by disposing a coating layer which is made by adding fluorine powder to a non-modified polyurethane resin (for example, Patent Document 4). With such techniques, the surface is made resistant to abrasion due to rubbing in a very short time and also resistant to wearing, because the surface is made more slippery. Nevertheless, the easiness of forming recessed scratch is not solved and the surface touch is limited only to a slippery touch.
Further, the improvement of the wear resistance and abrasion resistance by adding polyurethane fine particles to the polyurethane resin for forming the coating film has been proposed (for example, Patent Document 5). In the proposed technique, a coating composition prepared by adding spherical polyurethane fine particles having an average particle size of 1 to 800 μm to the binder resin such as polyurethane resin is used. Since the spherical polyurethane fine particles are fixed by the binder resin, a number of spherical polyurethane fine particles are exposed on the surface applied with the coating composition. As a result, the fine roughness of the surface reduces the contact area with an object, to obtain a good wear resistance and also improve the abrasion resistance in some extent. However, the effect of the proposed technique is obtained only in case of the appearance of suede-finished leather-like sheets and the surface touch is limited to rough feeling. If the fine particles exposed to the surface are easy to fall off, the abrasion resistance may be rather reduced, and the easiness of forming recessed scratch is not eliminated.
[Patent Document 1] JP 7-150479A
[Patent Document 2] JP 4-333674A
[Patent Document 3] JP 2000-248472A
[Patent Document 4] JP 10-251978A
[Patent Document 5] JP 2001-342434A